The increased demand for weight reduction and manufacture of complex shapes with tube hydroforming (THF) requires more accurate material models to achieve highly reliable results from simulations. Conventional flow behaviours of materials implemented in commercial finite element (FE) software are not capable of describing accurately the material behaviour after localization. Utilization of the stress-strain data after necking can improve the quality of the FE analysis by employing realistic data for extrapolating the stress-strain curve. In this paper, the objective was to develop a simple tool that can determine the true stress-strain curve after necking in order to predict the material behaviour of aerospace alloys such as stainless steel 321 (SS321). Standard uniaxial tensile tests (ASME E8M-04) were performed to determine the true stress-strain before necking. A weighted-average method and a new hardening equation were proposed to extend the curve after necking.